GNOME Summit is a three-day hackfest for GNOME developers and contributors.
It is not primarily aimed at users or new contributors, but if you want to jump right into the deep end, it's a fantastic way to meet everyone and get involved. Unlike traditional conferences, the Boston Summit is all about getting developers together and getting things done. While there are some non-hacking sessions, they are geared heavily towards many-to-many, interactive discussion and planning, rather than one-to-many presentations.